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| | Re: PH won't go down.
If there is a greater concentration of hydrogen ions [H+] vs. [OH-] (hydroxide ion concentration), the water is acidic and has a pH lower than 7. When [H+] is lower than [OH-], the water is basic and has a pH higher than 7. When [H+] = [OH-], the pH is neutral, or 7, exactly.
So, pH is simply the inverse log of the hydrogen ion concentration, denoted as:
pH = -log[H+]
Once you begin to consider Lewis Acids/Bases, pH becomes a little more complex, but the above is more than enough for most aquarists. The most important thing to remember is that you should never allow the pH to change rapidly in your tank -- limit any changes to .1 increments, if possible. Since pH is a logarithmic function, you can see that changing pH by a degree in either direction is actually a 10x change in relative acidity.
It's OK to manage by the numbers.
KH is Carbonate Hardness or Alkalinity and consists primarily of bicarbonate (HCO3-) (and other buffers).
Unless you're doing something special with your tank (biotope, peat, Reverse Osmosis, etc.), your KH should be around 5.
GH is General Hardness and consists specifically of the amount of calcium (Ca+2) and magnesium (Mg+2) in the water and generally does not directly affect pH. If you are manipulating GH by adding calcium, lime, or magnesium to your tank in some inorganic form, you may increase the KH by adding carbonate resources to the water column. As calcium carbonate (hard white precipitate - CaCO3) forms on your hood and elsewhere, your GH and KH will drop, since both calcium and carbonate are being removed from the water column. The subtle alterations in GH that affect KH, in turn indirectly affect pH because of the KH changes.
Unless you're doing something special with your tank (biotope, cichlids, Reverse Osmosis, breeding, etc.), your GH will be determined by whatever calcium and magnesium you've got in your tap water. GH is also called "permanent hardness." If you have hard water, you generally know it, since you use a water softener in your house and will may also note the accumulation of CaCO3 on your water faucets and elsewhere (besides the basic water chemistry test).
Water that has a low GH is called "soft water." Water that has a high GH is called "hard water." Similarly, because of the relationship between GH and KH, hard water tends to have higher alkalinity (KH).
KH is your primary defense against the build-up of acidic materials in the tank, include nitric acid from nitrate accumulation (the end-product of the ammonia produced by your fish) and carbonic acid from dissolved CO2 from the atmosphere, respiration, or direct injection. So, KH prevents sudden drops in pH. The higher the KH, the higher (& more stable) the pH. The lower the KH, the lower (and more unstable) the pH.
If you allow the nitrates from processed fish waste to build up in your tank to very high levels in the presence of low KH, you can experience a tank pH crash. This is why (among other reasons) that it's a good idea to perform regular water changes. Also, if you inject CO2 in the presence of an insufficient KH (alkalinity), you can cause a pH crash. Rapid, wide swings in pH will stress or kill your fish.
There is some useful info about GH, KH, and pH in this older thread and elsewhere in the forum:
KH vs. GH - what's the difference?
This is just a quick overview, but it should address the question. Once again, it's OK to manage by the numbers. Pay particular attention to your KH, while developing an appreciation for and understanding of the interaction among GH, KH, and pH.
Note that if you use a Reverse Osmosis filter, it will strip all the hardness and alkalinity out of the water, so you must re-supplement with essential buffering and electrolytes in order to prepare the water for use in your tank.
Also, there is some confusion around the words alkalinity and alkaline. Alkalinity is the total pH buffering capacity of the water (KH). However, [/b]Alkaline[/b] solutions are Bases and have a higher [OH-]. So, it's best not to confuse the two.